This invention relates to the art of digital data radio frequency communication, and more particularly to new and improved modulation and detection processes for digital radio communication.
One area of use of the present invention is in a radio transceiver employed in a metropolitan area communications network, although the principles of the present invention can be variously applied. Direct digital synthesizers and phase lock loops have been employed in the generation of frequency modulated transmission signals. In a multi-state frequency shift key modulation scheme, a potential problem exists in that transitions between frequency states can be abrupt causing undesirable spectral splatter. In the reception of such signals, a drawback of the typically fast-acting automatic frequency control circuit is the potential loss of low frequency data.
It would, therefore, be highly desirable to provide a digital data radio frequency modulation process and apparatus wherein the reference signal transitions smoothly from one state to another. It would also be highly desirable to provide an automatic frequency control for a digital radio wherein carrier frequency errors are removed and the information is detected in a simple yet highly effective manner.